<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>demo_id()</title>
    <style>
        #blue{
            color: skyblue;
        }
    </style>
</head>
<body>
    <div id="blue">demo_Hello-World(id="()")</div>
    <p id="blue">text</p> <!-- id代码可以在CSS实现 但不是用来修饰的 -->
   
</body>
</html>